1. Field of the Invention
The present invention relates to electro-optical devices used in liquid crystal and other electro-optical display devices, and more particularly, to electro-optical devices having pixel electrodes and nonlinear pixel driver elements positioned on one of a pair of substrates between which a liquid crystal material is sandwiched.
2. Related Technical Art
Conventional electro-optical devices such as liquid crystal displays often employ metal-insulator-metal (MIM) elements as nonlinear interface or driver elements for each pixel area or electrode. A perspective view of part of a liquid crystal display device using a MIM element in this manner is illustrated in FIG. 1. A top view of a portion of the liquid crystal substrate supporting one MIM element is illustrated in FIG. 2, and an enlarged cross section of this MIM element is shown in FIG. 3.
In these figures, a pair of substrates 1 and 2 are positioned on each side of a liquid crystal layer 3 forming a laminated structure or sandwich. One or more scanning electrodes 4, which are typically made from a transparent layer of material such as ITO, etc., are formed on one substrate, here 1, and a plurality of pixel signal or control electrodes 5, also made from ITO, etc., are formed in a matrix on the other substrate, here 2. Also shown in these figures, is a first conductor 6, typically manufactured from tantalum (Ta), which is covered by an insulator 7 manufactured from a form of tantalum oxide (TaO.sub.x), and followed by a second conductor 8 made from chrome (Cr). The overlapping portions of the first and second conductors and the insulator create the active area of a desired MIM element for driving a pixel electrode.
The MIM structure illustrated in these figures is termed a "lateral MIM" structure because only the side surfaces of the first conductor 6 are used in conjunction with the second conductor 8 to form each pixel driver element, that is, as the active element area. In a lateral MIM, the an extermely surfaces of the first conductor 6, i.e., other than the surfaces making up or actively forming the MIM, are covered with an additional, thin, insulator 7b which forms an electrical barrier layer. The electrical resistance of the layer 7b is made larger than that of the thin insulator 7a residing on side surfaces of first conductor 6 to prevent the top surface from interacting with other conductors and functioning as part of a MIM element. This configuration facilitates the manufacture of elements having an extremely small surface area, and has shown to be effective for increasing the density and precision of liquid crystal displays and other electro-optical devices utilizing MIM elements.
In electro-optical devices using MIM and other nonlinear elements such as that described above, where the capacitance of the pixel driver element (MIM) is labeled C.sub.MIM and the capacitance of the liquid crystal pixel (LC) driven by the nonlinear element is labeled C.sub.LC, the voltage impressed on the pixel driver element V.sub.ON is expressed by the relationship: EQU V.sub.ON ={C.sub.LC /(C.sub.MIM +C.sub.LC)}.multidot.V
where V is the applied circuit voltage. The larger the impressed voltage, the better the nonlinear property of the MIM element is utilized and the resistance of the MIM element can be reduced.
Therefore, assuming that the pixel capacitance CLC remains constant, the driver capacitance C.sub.MIM must be made small for the MIM element to efficiently drive a pixel with respect to any impressed voltage value. Generally, it is believed desirable that the capacitance ratio C.sub.LC /C.sub.MIM be set equal to or greater than 10. However, when the driver capacitance C.sub.MIM is made small, that is, the size of the element is made small, the resistance of the driver element increases and the effective voltage impressed on the liquid crystal layer decreases when a pixel selection or scanning drive is performed.
What is needed is a method and apparatus for manufacturing an electro-optical device using nonlinear pixel driver elements, such as MIMs, exhibiting good display performance and drive efficiency without requiring the driver element capacitance to be extremely small.